Method and apparatus for the bonding of a contact element

ABSTRACT

Method and apparatus for bonding a contact element (17) on a substrate (20), in which the contact element is held by a connecting device designed, in particular, as a bonding head (11) and the contact element or the substrate or both are loaded with thermal energy, wherein 
     the contact element rests on the substrate when loaded with energy and a relative movement takes place between the contact element and the substrate, 
     reference energy issuing from an emission surface (28) and transmitted to the contact element (17) is measured during the relative movement as a reference value for the quality of the relative position of the contact element (17), and 
     the relative movement and the energy loading are interrupted when an adequate reference value is measured.

The present invention relates to a method and an apparatus for thebonding of a contact element on a substrate.

An apparatus and a method for the production of a laser diodearrangement is known from EP-A-356988, in which the exact relativepositioning between one end of an optical fibre and a laser diode takesplace in the molten state of connection solder as a function of a peakof the radiation energy transmitted from the laser diode into theoptical fibre. Devices which are independent from one another, namely apositioning manipulator and a soldering piston are used for positioningand connection by the known method.

BACKGROUND OF THE INVENTION

The object of the present invention is to provide a method and anapparatus of the type mentioned at the outset which simplify the bondingof a contact element on a substrate.

This object is achieved by a method and an apparatus having the featuresof the invention.

SUMMARY OF THE INVENTION

With the method according to the invention, the contact element is heldby a connecting device which is designed as a bonding head and is at thesame time used for the positioning and energy loading of the contactelement.

To enable the supply of energy or the interruption thereof to becontrolled particularly exactly, energy loading by a laser source hasproven particularly advantageous as the laser energy can be introducedinto the connecting region at a precisely defined point by means of anoptical fibre arrangement, and the introduction of energy by energyradiation allows substantially delay-free shut off without a buffereffect, as is the case, for example, with thermal conduction.

It is particularly advantageous if the method according to the inventionis applied during the coupling of an optical fibre to an element whichemits laser energy, for example, a laser diode, the laser energy outletsurface of the element arranged on the substrate serving as emmissionsurface and the contact element being provided by the optical fibre.Application of the method according to the invention allowssubstantially loss-free coupling of the optical fibre to the laserdiode. The laser energy which is emitted from the laser energy outletsurface of the laser diode and is introduced into the optical fibreprovided for coupling to the laser diodes is used as reference energy.The reference energy issuing from the optical fibre can be measured todetermine the optimum relative position, the optimum relative positionbeing obtained when the issuing energy reaches its peak. Therefore, theapplication of the method in the production of a diode laser arrangementwith optical fibre coupling allows efficiency of the diode laserarrangement barely attainable hitherto.

To allow simple pre-adjustment of the contact element, which correspondsto the optical fibre when applying the method to the production of adiode laser arrangement, relative to the emission surface on thesubstrate, the contact element cross section can be received at leastpartially in a positioning device. This positioning device determinesthe limits within which relative positioning of the contact element withrespect to the emission surface is possible and therefore avoids anexcessively large positional deviation between the contact element andthe emission surface which would unnecessarily lengthen the duration ofimplementation of the method.

In the case of a metallic design of the outer periphery of the contactelement, that is the optical fibre cladding, the positioning device canhave at least two contact metallization mounds known as so-called bumps.This type of positioning device has the advantage that the positioningdevice itself provides the connecting material required for the thermalconnection between the contact element and the substrate.

In particular when the method is used for the bonding of a contactelement designed as a wire conductor, it may prove advantageous tosuperimpose ultrasonic energy on the thermal energy for the energyloading of the contact element.

The features of the invention relate to an apparatus which isparticularly suitable for carrying out the aforementioned method.

With this apparatus there is provided a connecting device which isdesigned as a bonding head and is connected via an optical fibrearrangement to a laser source, the bonding head being provided with adevice, preferably designed as a vacuum device, for holding the contactelement on the bonding head. The apparatus also comprises an energymeasuring device which can be connected to the contact element formeasuring reference energy transmitted from an energy emission devicearranged on the substrate to the contact element. The apparatus alsocomprises a substrate carrier for receiving the substrate.

The relative movement between the contact element and the substratenecessary for carrying out the method according to the invention can bepermitted by appropriate mobility of the bonding head. However, it isalso possible to provide a substrate carrier which is correspondinglymovable, that is movable at least in one axial direction of theconnecting plane between the contact element and the substratetransversely to the direction of emitted reference energy. This enablesthe bonding head itself to be relatively simple in design and aconventional compound table or the like which allows extremely preciseadjustments in the axial directions of movement to be used for themovable substrate carrier arrangement.

The optical fibre arrangement in the bonding head can be broughtinternally up to an energy transmitting member of the bonding head ofwhich the external surface serves to load the contact element withpressure and heat. Such a design of the bonding head allows a build upof heat to be created at the interface between the transmission memberand the contact element, which can be used to connect the contactelement to the substrate, particularly when using, for the transmissionmember, materials having a thermal capacity which is as low as possible.

A design of the energy transmission member which is independent of thebonding head also allows the energy transmission member to be adapted tothe respective material constitution of the contact element and to beexchanged when different contact elements are used on the bonding head.The use of energy transmission members has the further advantage that,despite uniform application of pressure in the region of the connectingpoint on the contact element, direct contact between the optical fibrearrangement in the bonding head and the contact element is avoided,preventing the associated wear on the end cross section of the opticalfibre arrangement.

It can prove advantageous, particularly when a transmission member isinterposed between the optical fibre arrangement and the contactelement, if the bonding head is additionally loaded with ultrasonicvibrations so as to introduce ultrasonic energy into the connectingpoint in addition to thermal energy.

A preferred embodiment of the method according to the invention andadvantageous embodiments of the apparatus for carrying out the methodare described in detail with reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a possible embodiment of an apparatus forcarrying out a variation of the method according to the invention.

FIG. 2 is an enlarged partial view of the apparatus shown in FIG. 1.

FIG. 3 shows a possible variation of the apparatus shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a first embodiment of a bonding apparatus 10 with a bondinghead 11 and a substrate carrier table 12 which is arranged beneath thebonding head 11 and is movable in the direction of the X, Y and Z axes.In a receiving bore in the bonding head 11 there is introduced a glassfibre line 13 which is provided with a radiation screen (not shown indetail) and is connected to a laser source 19. At its lower end, thereceiving bore 14 is enlarged to a vacuum chamber 15 from which thereissues a suction line 16 connected to a suction device (not shown indetail here). The vacuum chamber 15 serves to aspirate and hold acontact element designed here as an optical fibre 17 against a lowerface 18 of the bonding head 11.

On the substrate carrier table 12 there is arranged a substrate which isdesigned in this case as a carrier plate 20 and on which there arelocated a laser diode 21 rigidly connected thereto and two solderingbumps 22, 23 which are at a distance from the laser diode 21 andtogether form a positioning device 24 (FIG. 2). For holding the carrierplate 20 securely on the substrate carrier table 12, the substratecarrier table 12 is provided in a manner not shown in detail here withan aspiration device.

To the optical fibre 17 there is connected an energy measuring device 25which can be connected to the bonding head 11 or can form a separatedevice and is connected in terms of signalling on the one hand to thelaser source 19 and on the other hand to the substrate carrier table 12.

A possible variation of the method of bonding the optical fibre 17 tothe carrier plate 20 will be described in detail hereinafter withreference to FIGS. 1 and 2. The object of the connection, described hereby way of example, between the optical fibre 17 and the carrier plate 20is to produce a diode laser arrangement 26 in which laser radiationemitted from the laser diode 21 is to be introduced into the opticalfibre 17 and to be conveyed onward through it to an optional radiationoutlet point. It is important for a radiation inlet cross section 27 ofthe optical fibre 17 to be orientated exactly with respect to anemission surface 28 of the laser diode 21 so that the introduction ofthe laser radiation into the radiation inlet cross section 27 can takeplace as far as possible without losses. Direct connection between theemission surface 28 and the radiation inlet cross section 27 should beavoided as any deformations occurring could lead to radiation losses. Itis important for the performance of the diode laser arrangement 26 forthe emission surface 28 of the laser diode 21 amounting merely to about2 μm in diameter in the case of the laser diode selected as an exampleto be orientated relative to the optical fibre 17 in such a way that itsprojection lies within the core cross section of the optical fibre 17which forms the radiation inlet cross section 27 and is about 5 to 6 μmin diameter in this embodiment.

The substrate carrier table 12 with the carrier plate 20 arrangedthereon is initially driven in the direction of the Z axis toward theoptical fibre 17 held on the bonding head 11 by vacuum in such a waythat the configuration shown in FIG. 2 is adjusted. Preadjustment ispermitted in that the optical fibre 17 is received with its crosssection partially between the soldering bumps 22, 23 of the positioningdevice 24. In the embodiment illustrated, the soldering bumps 22, 23have a substantially rectangular cross section, but can also have anyother design if desired. To orientate the optical fibre 17 relative tothe bonding head 11, the lower end thereof is substantially funnelshaped in design and receives the optical fibre 17 with its peripherybetween two duct edges 29, 30 of an optical fibre duct 31 simultaneouslyforming the orifice of the vacuum chamber 15 to the exterior. An endcross section 38 of the glass fibre line 13 is located a short distancefrom the optical fibre 17 but can also rest on it.

The laser source 19 is activated on the basis of the configuration shownin FIG. 2, which leads to an output of laser radiation from the endcross section 38 of the glass fibre line 13 and corresponding heating ofthe optical fibre 17 located therebelow and provided with a metalcladding 32 in this embodiment. The heat is conveyed via the metalcladding 32 into the soldering bumps 22, 23, causing them to soften. Thesoftening of the soldering bumps 22, 23 leads to release of the rigidpositioning, so movements of the substrate carrier table 12 togetherwith the carrier plate 20 arranged thereon are now possible in thedirection of the X axis. Pressure contact between the bonding head 11and the substrate carrier table 12 which is guided via the optical fibre17 is maintained during these movements so the positioning of theoptical fibre 17 in the direction of the Z axis is also maintainedrelative to the substrate carrier table 12.

During the relative movement between the optical fibre 17 and thesubstrate carrier table 12, the laser diode 4 is activated and emitslaser radiation via its emission surface 28 toward the radiation inletcross section 27 of the optical fibre 17. The portion of emittedradiation energy which enters the optical fibre 17 and is transmitted byit is measured by the energy measuring device 25 connected to theoptical fibre 17. When the radiation peak is detected, the relativemovement between the optical fibre 17 and the substrate carrier table 12is interrupted and the laser source 19 is switched off. Both are carriedout via appropriate signals conveyed from the energy measuring device 25to an adjusting device (not shown in detail) of the substrate carriertable 12 and the laser source 19.

The switching off of the laser source 19 which can be carried outconstantly or intermittently and the interruption in the relativemovement during the softened state of the soldering bumps 22, 23 resultsin quasi freezing of the optimum relative position, defined by themeasured radiation peak, of the radiation inlet cross section 27 of theoptical fibre 17 with respect to the emission surface 28 of the laserdiode 21. After solidification of the connection between the carrierplate 20 and the optical fibre 17, an extremely powerful laser diodearrangement is created. It will be appreciated that correspondingoptimum positioning in the direction of the Y axis is possible in thesame manner as optimization of the relative position of the opticalfibre 17 with respect to the laser diode 21 in the direction of the Xaxis.

To connect an optical fibre which is not provided with metallic claddingbut, for example, with acrylic cladding to the carrier plate 20, thepositioning aids of the positioning device 24 designed as solderingbumps 22, 23 in the foregoing example can be formed from a correspondingplastics material which allows thermal connection to the optical fibre.It is similarly possible to apply the method described hereinbefore in avariation not only for the production of diode laser arrangements butquite generally whenever positioning of a contact element on a substratewhich is as accurate as possible is to be effected. It is then possibleto use, not a laser diode, but an energy emitting device of a quitegeneral type, for example a current-carrying conductor with a contactcross section, as emission surface. The electric current measured in thecontact element can then also be measured, for example, in relation tothe introduced electric current as a reference value for the correctrelative positioning. A bonding apparatus 39 of the type shown in FIG. 3can then also be used. In a modification of a known wedge-bond method,the energy required for the thermal connection is introduced as in theabove-described embodiment via a glass fibre line 13 into a bonding head33 modified in comparison with the aforementioned embodiment.

In contrast to the above-described method and the apparatus used for it,the bonding head 33 is provided with an energy transmitting member 34which is arranged between the end cross section 38 of the glass fibreline 13 and the periphery of the contact element designed here as a wireconductor 35. In the embodiment illustrated here, the substrate isprovided by a chip 36 having a soldering bump 37 for connection to thewire conductor 35. A non-melting partial area of the soldering bump 34can be provided as an emission surface in the embodiment illustratedhere.

It will be appreciated that the apparatus shown in FIG. 3 generallyaffords advantages for the connection of contact elements to a substratesurface which, in turn, can also be formed by a contact element,independently of the monitoring of the relative position between thecontact element and the substrate, as described in detail with referenceto FIGS. 1 and 2. The interposition of the energy transmission member 34between the end cross section 38 of the glass fibre line 13 and theperiphery of the wire conductor 35 with appropriate choice of materialfor the transmission member results in a build up of heat and thereforeto a local rise in temperature in the region of contact between theenergy transmission member 34 and the wire conductor 35, so theconnection can be made particularly effectively. This applies inparticular when loading with ultrasonic energy is superimposed on thethermal loading of the connecting point.

The invention claimed is:
 1. Method of bonding a contact element on asubstrate, in which the contact element is loaded with thermal energyand wherein the contact element rests on the substrate while energy isbeing loaded, comprising:preadjusting the position of the contactelement in front of an energy emission surface of an element emittingenergy by inserting the contact element's cross section between at leasttwo spaced contact metallization mounds arranged on the substrate;beginning loading of thermal energy in order to soften the metallizationmounds sufficiently to permit movement of the contact element relativeto the substrate; moving the contact element and the substrate relativeto each other for a fine positioning of the contact element on thesubstrate during loading of thermal energy; issuing reference energyfrom said energy emission surface and transmitting the energy to thecontact element during the relative movement; measuring the referenceenergy during the relative movement as a reference value for the qualityof the position of the contact element; and halting the relativemovement and the energy loading when an adequate reference value ismeasured, wherein the contact element is held by a bonding head which issimultaneously used for the fine positioning and energy loading of thecontact element.
 2. The method of claim 1 further comprising thepreliminary step of arranging said metallization mounds on saidsubstrate in front of said energy emission surface, providing a guidefor preadjusting the position of said contact element.